1. Appendix
One appendix is filed with this application. Appendix A is a software code listing comprising a total of 5 pages.
2. Field of the Invention
This invention relates to a cryogenic refrigerant apparatus for providing a fluid at low temperatures and, more particularly, to such an apparatus which permits such low temperatures to be achieved in an efficient manner even when the size of the apparatus is reduced in scale.
3. Background Information
Two well known techniques have been suggested for use in achieving low temperature or cryogenic operation, particularly using helium as a fluid, for example. One approach is referred to as a Collins Cycle (or alternatively referred to as a multi-stage Claude Cycle). The Collins Cycle is used to provide refrigeration or liquefaction at "liquid-helium" temperatures. The Claude Cycle is used to provide refrigeration or liquefaction at higher temperatures using fluids such as nitrogen. Improvements and modifications to this basic technique have also been described in U.S. Pat. Nos. 2,607,322 and 3,438,220, for example, issued S. C. Collins on Aug. 19, 1952 and Apr. 15, 1969, respectively.
In such approach, high pressure fluid from a compressor is passed through a heat exchanger and introduced, via a high pressure valve, into an expansion engine comprising a chamber having a movable member such as a piston positioned therein. When the fluid is so introduced, the piston moves within the chamber to form an expansion volume, the expansion of the fluid causing the heat energy to be transferred therefrom via the performance of mechanical work, as on a crank shaft, for example, connected to the piston. In the expansion operation, the temperature and pressure of the fluid are reduced considerably. The fluid is then conveyed via a low pressure valve from the expansion volume to a space to be cooled, for example, and then back to the compressor in a counter current flow through the heat exchanger.
While the Collins Cycle technique is effective when used for relatively large-scale production of low temperature helium, for example, it has been found to be difficult to scale down the apparatus size when a smaller system is required and still retain the low temperature effectiveness thereof.
Another approach used in the art to achieve low temperature operation is often referred to as the Gifford-McMahon Cycle Technique, an approach that has sometimes been proposed as effective when used for such smaller scale systems. The Gifford-McMahon Cycle is commonly used in single and multiple stage configurations. A multi-stage Gifford-McMahon Cycle, however is generally incapable of producing liquid-helium temperatures with conventional regenerator materials, as will be discussed hereinbelow. Refrigeration in a Gifford-McMahon operation results from a difference in enthalpy between the entering high pressure stream and the exiting low pressure stream. A basic description of the Gifford-McMahon operation is set forth in U.S. Pat. No. 3,045,436, issued on Jul. 24, 1962 to W. E. Gifford and H. O. McMahon. Other apparatus using Gifford-McMahon principles of operation are also described, for example, in U.S. Pat. Nos. 3,119,237 and 3,421,331 issued on Jan. 28, 1964 and Jan. 14, 1969 to W. E. Gifford and J. E. Webb, respectively.
In such systems, no heat energy is transferred from the expanding fluid through the performance of mechanical work external to the refrigerator. While a movable displacer element is periodically moved within the apparatus to provide for an expansion chamber, such element is not arranged so as to produce mechanical energy exchange. Rather, multiple confined fluid volumes are balanced so as to act in conjunction with one another so that compression and expansion are selectively controlled using inlet and exhaust valves at room temperatures and a net refrigeration is produced at one or more points in the system.
In such an approach, the confined fluid volumes on either end of the displacer are connected by a heat exchange passage called a thermal regenerator, as mentioned hereinabove. Those skilled in the art will recognize that the term thermoregenerator refers to bi-directional heat exchangers which often have been selected for relatively small scale refrigeration systems due to their mechanical simplicity, as will be discussed in greater detail hereinbelow. Thus in this Gifford-McMahon configuration, the regenerator undergoes the same pressure cycling as the confined fluid volumes. In this configuration, the heat energy is normally fully stored for a half cycle in the regenerator matrix, which requires the regenerator matrix to have a relatively large heat capacity. In totally regenerative cycles, such as in the Gifford-McMahon approach, the pressure ratio is effectively limited by the gas volume in the regenerator, which must be large enough so that the low pressure flow pressure drop through the regenerator matrix is not excessive. A similar cycle that employs regenerative heat exchangers is known as the Sterling cycle.
Common regenerator materials have a heat capacity that diminishes at very low temperatures. For this reason, early Gifford-McMahon Cycle machines alone have not been capable of producing cooling at liquid helium temperatures, even when multiple stages are used. To reach liquid helium temperatures, a second thermodynamic cycle such as a Joule-Thomson Cycle was generally used in parallel with the Gifford-McMahon Cycle. The Joule-Thomson Cycle consists of a pre-cooling counterflow heat exchanger and an expansion valve (commonly referred to as the Joule-Thomson expansion valve). Neither the Gifford-McMahon nor the Joule-Thomson cycles were capable of reaching liquid helium temperatures independently. The Gifford-McMahon stages provided pre-cooling of the helium gas in a counterflow heat exchanger of the Joule-Thomson Cycle in preparation to expand the gas over the Joule-Thomson expansion valve. This combined cycle configuration is capable of producing cooling at liquid helium temperatures. However, integrating these two cycled configurations is undesirable for at least two reasons. First, mechanically combining the two configurations is somewhat cumbersome, especially during manufacture. Second, the optimal mean cycle pressures and pressure ratios for the two cycles are not compatible, which requires a special compressor configuration.
More recently, small scale Gifford-McMahon cycle machines have been developed that may produce one Watt or less of cooling capacity at liquid helium temperature. These machines achieve such performance by use of rare earth materials (such as erbium three nickel) as the regenerator material, such as developed by Toshiba of Japan. While these machines may operate satisfactorily in some applications, they provide relatively low efficiency and generally have been limited to a capacity of one Watt or less at liquid helium temperatures.
Thus, existing technology for cryogenic cooling on a relatively small scale i.e., for cooling of electronics and the like, is either overly expensive, to large and/or heavy, or inefficient. There is a need for a compact, lightweight, efficient, and low cost cryocooler which can provide approximately 10 Watts or less of cooling capacity at about 10 degrees Kelvin (K) or less for use with terrestrially deployed electronic devices such as superconductors, digital circuits and subsystems, Josephson voltage standards, and long wavelength infrared imaging cameras. Previous attempts to reduce the cost of small cryocoolers have aimed at simplifying the process (e.g., pulse-tube refrigerators, thermoelectrics) and/or reducing system or mechanical complexity. Disadvantageously, such simplifications have failed to attain or even approach the efficiency levels of the large scale systems that employ more complex cycles.
Large scale machines, such as those used in the production of liquid helium, are generally based on the Brayton or Collins Cycles. The cycles may be characterized as employing constant pressure, quasi-steady, recuperative heat exchange between the high and low-pressure gas streams, thereby requiring a two-stream (i.e., recuperative) heat exchanger. This compares with the variable pressure, periodic, regenerative heat exchange of the Sterling or Gifford-McMahon Cycles, which as discussed hereinabove, are typically used in small scale machines, due in part, to the relatively less complex and less expensive single-stream (i.e., bi-directional) regenerator. The small scale Brayton and Collins Cycle machines also require valved expanders and compressors, whereas the simpler regenerative cycles are valveless or have only warm valves (i.e., in the Gifford-McMahon Cycle). Heretofore, the relative complexity of these valved recuperative cycles has ruled out their use in low-cost machines utilized for the above-described, relatively small-scale applications of approximately 10 Watts of cooling capacity at about 10 degrees K or less. A comparison of thermodynamic efficiency at liquid nitrogen temperature (77K) shows however that the large scale, high efficiency machines based on the Brayton and Collins Cycles routinely achieve 25 percent of Carnot-efficiency at 77K (10 percent of Carnot-efficiency at 4K). The relatively simple machines such as the Gifford McMahon and/or pulse-tube systems typically achieve less than 10 percent of Carnot-efficiency at 77K.
Thus, a need exists for an improved cryocooler, which provides convenient scalability to small scale applications while maintaining relatively high energy efficiency associated with larger scale systems.
Throughout this application, various publications and patents are referred to by an identifying citation. The disclosures of the publications and patents referenced in this application are hereby incorporated by reference into the present disclosure.